Trump marked the 16th anniversary of the worst act of terrorism on U.S. soil in a ceremony at the White House, with a moment of silence at 8:40 a.m., the time when a first airliner hijacked by al-Qaida terrorists crashed into the World Trade Center in New York. A second jet hit the second WTC tower 23 minutes later, and the huge skyscrapers collapsed in flames and smoke soon afterwards.

Trump will also attend a remembrance ceremony later Monday morning at the Pentagon, which was hit and badly damaged by another of the four commercial jets seized on that fateful day. Early Monday, an American flag was unfurled down the side of the building.

Vice President Mike Pence will travel to Shanksville, Pennsylvania, where another hijacked plane crashed into a field. It is believed passengers tried to regain control of the aircraft once they realized their knife-wielding captors intended to steer it toward Washington for another attack, aiming for either the Capitol or the White House, and that the hijackers crashed the plane rather than surrender.

Everyone aboard all four hijacked aircraft died on September 11, 2001. Including mass casualties in New York City and a heavy loss of life at the Pentagon, the attacks killed a total of about 3,000 people.

The Islamist militant named by U.S. investigators as “the principal architect” of the 2001 attacks,” Khalid Sheikh Mohammed, was captured in Pakistan early in 2003 and is held at the U.S. military prison in Guantanamo Bay, Cuba. The September 11 plot had been approved in advance by al-Qaida leader Osama bin Laden, who was shot to death by U.S. commandos in May 2011, during a raid on a walled compound in Pakistan where he was hiding